1. Field of the Invention
The invention provides a method and apparatus for sealing a rotary joint formed between a fixed element and a rotatable element. In particular, a rotary joint formed between a fixed member a rotating member includes a continuous annular clearance gap formed between the fixed member and the rotating member. The clearance gap is sealed to prevent environmental elements from penetrating the clearance gap by providing a combination of sealing elements and features, including a labyrinth seal configuration, a wiper seal and a conventional gas-tight fluid seal within the clearance gap. In addition, the fixed member include features for repelling damaging external environmental elements thrust upon it by wind or wave action.
2. Description of the Prior Art
Turret mounted airborne camera systems are used in military and public safety for search and rescue, surveillance and reconnaissance. Turret mounted airborne camera systems are also used by the news media for generating broadcast quality images. Airborne camera systems comprise a sensor payload having one or more radiation detecting sensors with each sensor having an associated optical system for forming an image of a scene or object onto the sensors. The payload may include one or more radiation emitters such as a laser, for laser range finding, or another radiation source for illuminating a scene or object at a desired spectral range. In general, each sensor or emitter system of the payload has a limited field of view for receiving radiation from or emitting radiation to and the limited field of view may be adjustable in size by increasing or decreasing optical magnification. A central axis of the field of view defines a pointing direction.
In one prior art example of an airborne camera configuration, manufactured by FLIR SYSTEMS of North Billerica Mass., (the assignee of the present invention), the sensor/emitter payload is housed within a sealed spherical, or ball, housing that includes one or more sealed apertures for receiving or emitting radiation therethrough. (See FLIR product Ultra 8500. ) The payload includes a visible light intensifying or low light visible camera, an infrared camera, and a laser illuminator. The pointing direction of each payload element is substantially pointed at the same location in a far field of the sensor/emitter system. The ball housing is supported by a motorized turret that rotates the ball about two axes for directing the pointing direction onto a desired target area. The turret is configured to rotate the payload ball by about 180 degrees in elevation to direct the pointing direction from an axis normal to the ground to an axis directed at either horizon. The turret is also configured to rotate the payload ball continuously in azimuth to direct the pointing direction over a 360 degree range. The example airborne camera system is available for mounting on a helicopter or a fixed wing aircraft. It is usually the case that such an airborne camera system is carried on the underside of the aircraft with the pointing direction being generally directed toward the ground during operation. While airborne camera systems are usually well protected from airborne environmental hazards such shock, vibration, high speed airflow, rain, extreme temperature variations and the like, airborne camera systems have heretofore been designed with a downward facing turret and ball assembly.
Recently, the demand for increased surveillance in many areas has lead to the need for camera systems of similar design and capabilities to be mounted on land and sea vehicles as well as onto fixed structures. In early attempts to meet these demands, standard turret mounted airborne camera systems have been mounted onto ships, land vehicles and fixed structures for land and sea based missions. However, in land and sea based applications, it is has nearly always been the case that the camera system is mounted on the topside of the vehicle or fixed structure with its turret and ball assembly facing upward. However, using a standard airborne camera with its turret and ball assembly facing upward has proved to be problematic. In particular, land and sea based camera systems have been damaged by environmental elements penetrating seals and contaminating the payload ball and the turret motor drive systems. Users of upward facing turret and ball assemblies have experienced contaminate penetration failures caused by wind blown sand and other particulate matter as well as rain on land, and by wind and wave driven water and salt fog or mist at sea. One reason for these failures has been shown to be that the upward facing turret is particularly susceptible to contaminates collecting in and filling the upward facing annular clearance gap between a fixed turret base and a rotating turret member. In particular, the turrets upward facing 360 degree azimuth rotation mechanism is susceptible to contaminate penetration. While the azimuth rotation mechanism of prior art camera systems include a conventional magnetic fluid gas-tight seals in its annular clearance gap, applicants have found that the fluid of the gas-tight seals breaks down quickly in the presence of environmental contaminates that make contact with the magnetic fluid. Since the azimuth rotary joint faces downwardly in airborne cameras, contaminants were unable to collect in the clearance gap and contact the magnetic fluid. However, when the clearance gap faces upward, there is a need to provide additional sealing in the gap to prevent environmental contaminates from coming into contact the magnetic fluid of the gas tight seal.
In one prior art example of magnetic fluid seal used in combination with another seal, U.S. Pat. No. 4,890,940 by Schmidt et al. teaches a ball bearing that includes a gas tight fluid seal formed between one end of an inner and an outer race of the ball bearing. The fluid seal comprises a magnet circuit for providing a flux path in which a magnetic fluid is contained within a thin an annular clearance gap formed between the inner and outer races. The magnetic fluid forms a gas tight seal between an internal bearing environment and an external environment. Schmidt et al. teach a pre-seal positioned between the fluid seal and the internal bearing environment for preventing bearing lubricant from mixing with the magnetic fluid and altering its magnetic properties. The pre-seal, taught by Schmidt et al., utilizes the continuous high rotational velocity of the bearing inner race combined with a particular shape of a pre-seal wall facing the inner race to generate a centrifugal force local to the pre-seal wall facing the inner race and the centrifugal force pushes lubricant away from the pre-seal. The pre-seal is also configured to provide an annular liquid trapping groove for trapping magnetic fluid that may become separated from the gas seal. While the pre-seal of Schmidt et al. includes features that help to keep the internal bearing lubricant separated from the magnetic fluid of the gas seal, the pre-seal of Schmidt et al. still leaves an unsealed annular gap between the rotating inner race and the fixed outer race and this gap may be penetrated by contaminants. Moreover, the pre-seal, of Schmidt et al., requires continuous high speed rotation to generate the centrifugal force necessary to repel bearing lubricant from the seal area. However, the azimuth rotation of the present invention does not rotate continuously nor does it rotate at high speed so that a pre-seal of the type taught by Schmidt et al. would be ineffective. Moreover, the pre-seal of Schmidt et al. is particularly designed for one contaminate, the bearing lubricant, which has known properties and the seal may not be affective in sealing the gap from a variety of contaminants having different properties. Accordingly, there is a need for a pre-seal that is usable for preventing a variety of contaminates having different properties from reaching a magnetic fluid used in a gas-tight magnetic seal when the rotation of the rotary joint is a not continuous and not at a high velocity.